1. Field of Invention
This invention relates to a capacitive type converter device using a detection unit comprising a variable condenser having a capacitance between electrodes which varies depending on the distance between the electrodes, and a reference condenser having a fixed capacitance which is independent of the varying capacitance. The distance between the electrodes of the variable condenser may be varied by such factors, as pressure, pressure difference, which are to be measured.
2. Description of the Prior Art
FIG. 1 depicts an example of a conventional converting device of the capacitive type, wherein the detection unit 10 comprises a variable condenser 11 having capacitance C1 which varies in response to a value to be measured (such as pressure) and a reference condenser 12 having a capacitance C2 which is fixed. By means of an oscillation output of an oscillator 20, alternating current i1 and i2, responsive respectively to these capacitances C1,C2, are caused to flow through these condensers.
Alternating current i1 is rectified and smoothed by a detecting circuit 31 comprising rectifying diodes D1 and D2, smoothing condenser Cf1, and resistor R1, which generates across resistor R1 a DC voltage E1 corresponding to capacitance C1, which is represented by the following equation: EQU E1=f.multidot.eB.multidot.R1.multidot.C1 1
wherein f is the oscillation frequency of oscillator 20, and eB is the amplitude of oscillation output of oscillator 20.
Alternating current i2 is smoothed by a detecting circuit 32 comprising rectifying diodes D3 and D4, smoothing condenser Cf2, and resistor R2, which generates across resistor R2, a DC voltage E2 corresponding to capacitance C2, which is represented by the following equation: EQU E2=f.multidot.eB.multidot.R2.multidot.C2 2
By controlling the oscillation output of oscillator 20 by means of a control circuit 40, using a differential amplifier A, so that the DC voltage E1 becomes equal to a reference voltage Es, influence due to the amplitude eB of the oscillation output is eliminated and DC voltage E2 generated across resistor R2 satisfies the relation represented by the following equation: EQU E2={(R2.multidot.C2.multidot.Es)/(R1.multidot.C1)} 3
Detection unit 10 has a structure or configuration such as depicted in FIG. 2, wherein arranged opposite to an insulating member 13, such as glass, is a silicon substrate 14 which is formed partially with a diaphragm section 14a. On silicon substrate 14 there is provided a common electrode 15. A portion of the common electrode on diaphragm 14a functions as a movable electrode 15a and the other portion functions as a fixed electrode 15b. On insulating member 13 there are provided different fixed electrodes 16 and 17 concentrically and opposite to common electrode 15. The outside of silicon substrate 14 is maintained, for example, at atmospheric or ambient pressure.
Accordingly, in response to a pressure P (see arrow in FIG. 2) to be measured, for example, applied through an opening 13a of insulating member 13, diaphragm section 14a is deformed or displaced. In response to such displacement, capacitance C1 of variable condenser 11 existing between the movable electrode 15a and fixed electrode 16 is caused to be varied. On the other hand, capacitance C2 of reference condenser 12 existing between fixed electrode 15b and fixed electrode 17 is maintained unchanged and independent of the displacement of diaphragm section 14a.
Capacitance C1 of variable condenser 11 relative to the extent of displacement X of movable electrode 15a is given by the following equation: EQU C1=C0.multidot.{d/(d+X)} 4
wherein CO denotes the initial capacitance at X=0, and d denotes the reference distance between movable electrode 15a and fixed electrode 16 (the distance at X=0).
If capacitance C2 of reference condenser 12 is selected to be equal to initial capacitance C0, DC voltage E2 becomes EQU E2=(R1/R2).multidot.{1+(X/d)}.multidot.Es 5
Thus, DC voltage E2 which is proportional to the distance between the electrodes is obtained at an output terminal Eout.
However, the prior art device has certain disadvantages. For example, the variable condenser 11 includes a stray capacitance Cs1, existing between fixed electrode 16 and a reference point, a stray capacitance Cs2, existing between movable electrode 15a and the reference point, and a stray capacitance Cs3, existing parallelly between movable electrode 15a and fixed electrode 16. Although stray capacitance Cs2 presents no special problem if the output impedance of oscillator 20 is low, the other stray capacitances Cs1 and Cs3 tend to impair the linearity of measurement.
As another example of a disadvantage with the prior devices, in case capacitance C1 of variable condenser 11 is very small, for example, of the order of 10 pF, capacitance C1 is influenced by a parallel capacitance CD, of the order of 2 pF of rectifying diodes D1 and D2 and by a forward voltage ED (=0.6 v) of these diodes.
A further disadvantage to the prior art is a very small displacement of variable condenser 11 cannot be converted accurately into a DC signal due to adverse influence of temperature variation of parts (other than the rectifying diodes).